(1) Field of the Invention
The present invention relates to a storage service in the digital broadcast or the like, whereby received scrambled content is stored and then descrambled as required. More specifically, the present invention relates to technology for improving performance of the service in some particular reproduction modes.
(2) Description of the Related Art
In the current pay satellite digital broadcast system, a viewer contracts with a broadcast provider to pay some amount of charges to the broadcast provider for viewing programs on selected channels.
Since a broadcasting satellite broadcasts on a number of channels, programs on the channels for which a viewer does not make subscribing contracts are also broadcast to the viewer. The receiver owned by the viewer, then, confirms the presence/absence of the subscribing contract for each channel and receives only programs on the channels allowed to be viewed. Such technology for selecting channels to receive programs thereon is called Conditional Access (abbreviated as “CA”).
On the other hand, the transmitter scrambles content including audio and visual data of a pay broadcast program to be broadcast one each channel, where content per predetermined unit is scrambled with a scrambling key different for each predetermined unit of contents. The transmitter, then, sequentially transmits the scrambled content accompanied with an Entitlement Control Message (ECM) containing this scrambling key and the scrambling key for content in the successive predetermined unit. This ECM is encrypted so as to be interpreted only by the receivers owned by subscribers to the channel.
Note that the scrambling key used for scrambling data functions as a descrambling key as well for descrambling the data.
Each receiver comes with an IC card used for the receiver only, to which an identifying number associated with the subscriber is assigned. The IC card has a CPU and a memory within it to store contract conditions and software for executing CA processes. The data relating to the CA processes is completely digitized and contained in the IC card, which makes unauthorized viewing difficult and therefore provides high levels of security.
Meanwhile, the service called a “storage service” is scheduled to become operational targeted for those who have not yet subscribed to a channel so that, once storing scrambled content with ECM in the recording media in the receivers owned by them, they pay some amount of charges for viewing the content to become subscribers, which enables them to decode the ECMs so as to reproduce the stored scrambled content.
In such a service, when reproducing content in the normal reproduction mode, the receiver acquires content in the first predetermined unit, and decodes the accompanied ECM to extract the scrambling keys for content in the first and the second units. Then, the receiver descrambles the content in the first unit using the scrambling key exclusive to the content. As for the content in the second unit or later, the receiver sequentially descrambles content in each unit using the scrambling key obtained in the process for the preceding unit.
However, when reproducing content by the storage service in the particular reproduction modes such as a fast-forward reproduction mode and a fast-reverse reproduction mode, the receiver cannot descramble content in each unit using the scrambling key obtained in the process for the preceding unit, because the order of the reproduction becomes different from that in the normal reproduction mode. Therefore, the receiver has to, each time acquiring content in a predetermined unit, decode the accompanied ECM to obtain a scrambling key and descramble the scrambled content using the key, which makes it difficult to realize a sufficient performance level of the particular reproduction modes, such as fast-forward speed.